I have tried commercial "chin straps" and agree that they are mostly junk. The best thing I have found is strips of Velcro ONE-WRAP Cable Management, Self Gripping Cable Ties attached to both sides of my lower mask strap. Adjust under the chin to whatever works for you. No more mouth breathing from me since using them.
I use the ResMed Mirage Quattro full face mask.
If you're a mouth breather and using a nasal mask it's unlikely that any chin strap is going to solve your problem. I think it would be better to search for a FFM that works for you. Perhaps a minimal one like the Amara view would work for you. I'm a side sleeper and successfully used it until I switched to a nasal mask.